Many devices designed to generate color images from rasterized image data cannot fully utilize the color information present in the image data. Some devices have technical limitations that prevent them from generating the full range of colors represented in the image data. For example, in some rasterized image data a respective pixel's color information includes data for three component colors such as red, green and blue, and the intensity of each of the three colors is represented by eight bits, allowing for 256 possible levels of intensity for each component color. If an image represented in this way is displayed on a device capable of generating only eight different levels of intensity for each component color, a significant amount of color information is lost. In such cases, the color information is typically quantized using a quantization method such as uniform quantization, allowing the device to display an image using fewer colors than are present in the image data. In addition to eliminating color information from the image, quantization methods frequently result in the appearance within the output image of visual artifacts. For example, an effect known as contouring may occur if the quantization of the image data causes visible transitions between color levels.
The need to optimize the color capabilities of a display device arises not only in the display of two-dimensional (“2-D”) image data but also in the presentation of three-dimensional (“3-D”) image data such as video. As used herein, the term “3-D image data” refers to digital image data in which the color of a pixel is a function not only of two spatial coordinates but also of a time coordinate. When 3-D image data is presented on a display device such as a LCD or computer monitor, a single pixel on the screen may display different colors as time progresses. An array of pixels having the same time coordinate is commonly referred to as a “frame”. Multiple frames are typically displayed, or “cycled,” in rapid succession to generate a moving image.
One technique used to expand the range of colors achievable by a device with limited color capability is known as 3-D screening. Generally, screening techniques exploit characteristics of the human eye to expand the color output of a device beyond its inherent color capabilities. If the human eye views a pattern of sufficiently small dots having different colors, the viewer generally does not perceive the colors of the individual dots but instead perceives a color approximately equal to the average color of the dots in the pattern. The human eye's ability to perceive the average color operates spatially, when the eye views a small array of dots on a display device, and over time, when the eye views an array of pixels whose colors are rapidly changing. In either case, the eye typically perceives a color that is approximately equal to the average color of the pixels viewed. The color perceived by a viewer is referred to as the “effective color output.” Screening techniques that are used to generate moving images typically construct three-dimensional arrays of values, or “screens,” which can be used to generate within an array of pixels a pattern of colors with a desired effective color output. These techniques enable a device to expand the number of colors it can display by generating a display that appears to have additional color levels intermediate to those defined by the device's physical characteristics.
A common 3-D screening technique known as frame rate modulation (“FRM”) turns a pixel alternately on and off across multiple frames, producing an effective color output that is approximately equal to the average color measured over a series of frames. The term “refresh rate” refers to the frequency at which the output of a single pixel alternates between different color levels. Increasing the refresh rate can enable a device to display a greater number of intermediate colors and can provide greater control over the effective color output.
An additional factor that influences the perceived quality of the color output is the perceived uniformity of the color pattern. It is preferable in presenting 3-D image data to use a screen that produces color patterns having a high degree of uniformity. Known FRM techniques are inconsistent in their ability to produce uniform patterns.
There is a need to develop an improved 3-D screening technique that increases the number of color levels that a device may generate in presenting 3-D image data and provides greater control over the uniformity of the color pattern displayed, while minimizing the appearance of artifacts.